Mobile communication system, base station apparatus and mobile station apparatus

ABSTRACT

The time spent in handover is shortened when a mobile station apparatus performs random access for handover. In a mobile communication system where a mobile station apparatus uses any signature of a beforehand determined signature group between the mobile station apparatus and a base station apparatus in random access, the base station apparatus selects a signature used by the mobile station apparatus in random access performed at the time of handover, and the mobile station apparatus performs handover using the signature selected in the base station apparatus and judges the presence or absence of transmission and reception of a handover complete message and contention resolution corresponding to the type of signature selected in the base station apparatus.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/601,507, filed on Nov. 23, 2009, which is a continuation ofInternational Application No. PCT/JP2008/059015, filed on May 16, 2008.The International Application claims priority to Japan PatentApplication No. JP2007-137662, filed on May 24, 2007. Theafore-mentioned patent applications are hereby incorporated by referencein their entireties

TECHNICAL FIELD

The present invention relates to a mobile communication system, basestation apparatus and mobile station apparatus using a cellular radioscheme.

BACKGROUND ART

Currently, in 3GPP (3rd Generation Partnership Project), the W-CDMAsystem has been standardized as a 3G cellular mobile communicationsystem, and its service has been started sequentially. Further, HSDPA(High Speed Downlink Packet Access) with the communication speed furtherincreased has also been standardized, and its service is being started.

Meanwhile, in 3GPP, evolution in 3rd Generation Radio Access (EvolvedUniversal Terrestrial Radio Access: hereinafter, referred to as “EUTRA”)has been studied. As downlink in the EUTRA, an OFDM (OrthogonalFrequency Division Multiplexing) system is proposed. Further, proposedas uplink in the EUTRA is a DFT (Discrete Fourier Transform)-spread OFDMtype single carrier communication system.

As shown in FIG. 22, the uplink of EUTRA is formed of an uplink pilotchannel UPiCH, random access channel RACH, and uplink scheduling channelUSCH (for example, see Non-Patent Document 1).

The uplink random access channel RACH of E-UTRA contains anon-synchronized random access channel and synchronized random accesschannel. Herein, a band of 1.25 MHz is used as a maximum unit of thenon-synchronized random access channel. Then, for example, as shown inFIG. 23, a plurality of channels for access is prepared, and configuredto be able to respond to a number of accesses.

Among intended purposes of the non-synchronized random access channel,it is desirable to synchronize a mobile station apparatus (hereinafter,referred as a “mobile station”) and base station apparatus (hereafter,referred to as a “base station”). Further, it is considered that amobile station transmits several-bit information to request schedulingfor allocating radio resource and the like so as to decrease theconnection time between the mobile station and base station. Meanwhile,the intended purpose of the synchronized random access is to make ascheduling request (for example, see Non-patent Document 2).

In the non-synchronized random access, only a preamble is transmitted toacquire synchronization. This preamble contains a signature that is asignal pattern indicative of information, and by preparing a few tens ofkinds of signatures, it is possible to designate several-bitinformation. Currently, it is anticipated that 6-bit information istransmitted, and that 64 kinds of signatures are prepared.

In the 6-bit information, it is expected that 5 bits are assigned arandom ID, while remaining 1 bit is assigned a reason of random access,downlink path-loss/CQI (Channel Quality Indicator) and the like (forexample, see Non-patent Document 3).

FIG.24 is a sequence chart to explain an example of a conventionalprocedure of random access. In addition, FIG. 24 shows the procedure ofrandom access (non-synchronized random access) in the case of using anon-synchronized random access channel.

As shown in FIG. 24, in the conventional procedure of random access, amobile station first selects a signature based on a random ID, thereason of random access, downlink path-loss/CQI information and the like(step (hereinafter, abbreviated as “ST”) 2401). Then, the mobile stationtransmits a preamble (random access preamble) containing the selectedsignature on the non-synchronized random access channel (ST2402:Message1).

Upon receiving the preamble from the mobile station, the base stationcalculates a synchronization timing deviation between the mobile stationand base station from the preamble, and performs scheduling fortransmitting an L2/L3 (Layer2/Layer3) message (ST2403). Then, the basestation assigns C-RNTI (Cell-Radio Network Temporary Identity) to themobile station requiring C-RNTI from the random access reason, andtransmits a random access response including synchronization timingdeviation information (synchronization information), schedulinginformation, signature ID number and C-RNTI (ST2404:Message 2).

Upon receiving these pieces of information from the base station, themobile station extracts the response from the base station including thetransmitted signature ID number (ST2405). Then, the mobile stationtransmits an L2/L3 message with radio resources subjected to schedulingin the base station (ST2406:Message 3). Upon receiving the L2/L3 messagefrom the mobile station, the base station transmits a contentionresolution to judge whether a collision occurs between mobile stationsto the mobile station (ST2407:Message 4)(for example, see Non-patentDocument 3).

A problem of such random access is that a collision occurs in the casethat a plurality of different mobile stations selects the same signatureand random access channel. When a plurality of mobile stations selectsthe same signature and transmits the signature with a radio resourceblock having the same time and frequency i.e. on the same random accesschannel, a collision occurs in the preamble (ST2402) as shown in FIG.24.

When the base station cannot detect the preamble (ST2402) due to such acollision, the base station cannot send back the response (ST2404)including the synchronization information and the like to the mobilestation. In this case, the mobile station cannot receive the response(ST2404) from the base station, and therefore, needs to select asignature and random access channel again after a lapse of predeterminedtime to perform random access.

Meanwhile, when the base station can detect the preamble (ST2402), thebase station calculates L2/L3 message scheduling and synchronizationtiming deviation, and sends back a response (ST2404) to the mobilestation. However, a plurality of mobile stations receives the response(ST2404) from the base station. Therefore, the plurality of mobilestations transmits the L2/L3 message (ST2406) with radio resourcessubjected to scheduling, and as a result, the collision occurs in theL2/L3 message (ST2406).

When the base station cannot detect the L2/L3 message (ST2406) due tosuch a collision, the base station cannot send back the response(ST2407) to the mobile stations. In this case, the mobile stationscannot receive the response (ST2407) from the base station, andtherefore, need to select a signature and random access channel againafter a lapse of predetermined time to perform random access. Thus, whena plurality of mobile stations selects the same signature and randomaccess channel, the collision can occur, while when the collisionoccurs, the time up to ST2407 as shown in FIG. 24 is required at themaximum until the collision is detected.

Meanwhile, when a mobile station capable of executing such random accessis located in a position as shown in FIG. 25, handover is executed. Alsowhen handover is executed, the above-mentioned non-synchronized randomaccess is performed.

Described herein is an example of a procedure of random access at thetime of executing handover. FIG. 26 is a sequence chart to explain anexample of a procedure of random access at the time of executinghandover. In addition, as in FIG. 24, FIG. 26 shows the procedure ofrandom access in the case of using a non-synchronized random accesschannel.

As shown in FIG. 26, in the procedure of random access at the time ofexecuting handover, as a preparatory stage of handover, a mobile stationfirst measures radio signal conditions of adjacent base stations(ST2601). Then, the mobile station transmits the measurement result(measurement report) to a base station A that is a base station(hereinafter, referred to as a “local-base station” as appropriate)currently holding the mobile station (ST2602).

Upon receiving the measurement result from the mobile station, the basestation A selects an optimal base station from the measurement result(ST2603). In addition, herein, a base station B is assumed to beselected as an optimal base station. Then, the base station A transmitsa handover request message to the base station B that is a handoverdestination (ST2604).

Upon receiving the handover request message from the base station A, thebase station B assigns C-RNTI to the mobile station performing handover(ST2605). Then, as a response to the handover request, the base stationB notifies the base station A of a handover request acknowledge messageincluding the C-RNTI (ST2606).

Upon receiving the handover request acknowledge message from the basestation B, the base station A transmits a handover command messageincluding the C-RNTI to the mobile station (ST2607).

Upon receiving the handover command message from the base station A, themobile station acquires synchronization on downlink of the base stationB, and confirms a position of the random access channel from thebroadcast channel (ST2608). When the downlink synchronization isacquired, the mobile station selects one signature from among signaturessuch that the reason of random access is handover (ST2609). Then, themobile station transmits a preamble (random access preamble) containingthe selected signature to the base station B on the random accesschannel (ST2610:Message 1).

Upon detecting the signature from the preamble received from the mobilestation, the base station B calculates a synchronization timingdeviation, and performs scheduling of uplink for the mobile station totransmit a handover complete message (ST2611). Then, the base station Btransmits synchronization timing deviation information (synchronizationinformation), scheduling information and signature ID number to themobile station (ST2612:Message). In addition, in the case that therandom access reason is handover, the mobile station is beforehandnotified of C-RNTI, and therefore, the base station B does not transmitthe C-RNTI.

Upon receiving the information to the mobile station from the basestation B, the mobile station corrects the synchronization timingdeviation based on the synchronization timing deviation information(synchronization information) (ST2613). Then, the mobile stationtransmits a handover complete message with radio resources subjected toscheduling to the base station B (ST2614:Message 3). Upon receiving thehandover complete message from the mobile station, the base station Btransmits a contention resolution to judge whether a collision occursbetween mobile stations to the mobile station (ST2615:Message 4).

Thus, since random access is performed also at the time of handover, thecollision is inevitable, and it is feared that it will take much time tocomplete handover. To avoid the fear, proposals not to cause a collisionto occur in random access in handover have been made such that the basestation assigns a handover random access channel to other physicalresources and notifies the mobile station of using the handover randomaccess channel, and that the base station selects a signature forhandover to notify the mobile station to perform random access (forexample, see Non-patent Documents 4 and 5).

-   Non-patent Document 1: R1-050850 “Physical Channel and Multiplexing    in Evolved UTRA Uplink”, 3GPP TSG RAN WG1 Meeting #42 London, UK,    Aug. 29-Sep. 2, 2005-   Non-patent Document 2: 3GPP TR (Technical Report) 25.814,    V7.0.0(June 2006), Physical layer aspects for evolved Universal    Terrestrial Radio Access (UTRA)-   Non-patent Document 3: 3GPP TS (Technical Specification) 36.300,    V0.90 (March 2007), Evolved Universal Terrestrial Radio Access    (E-UTRA) and Evolved Universal Terrestrial Radio Access Network    (E-UTRAN), Overall description Stage 2-   Non-patent Document 4: R2-063082 “No-contention based handover    execution”, 3GPP TSG RAN WG2 Meeting #56 Riga, Latvia, Nov. 6-10,    2006-   Non-patent Document 5: R2-063225 “RACH Partitioning for Handover”,    3GPP TSG RAN WG2 Meeting #56 Riga, Latvia, Nov. 6-10, 2006

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

However, as described above, even in the case of using techniques not tocause a collision to occur, considerable time is spent in performingsteps from downlink synchronization and preamble (Message 1) of therandom access procedure up to contention resolution (Message 4) at thetime of executing handover. Particularly, in traffic with high real-timecharacteristics, the effect of data delay is significant.

The reason why random access is used in executing handover is that thebase station measures a deviation amount in transmission from the mobilestation so as to acquire uplink synchronization between the base stationand mobile station. Therefore, the preamble (Message 1) and preambleresponse (Message 2) are always required. The handover complete message(Message 3) is used to transmit a message of handover completion, andfurther used to specify the mobile station with C-RNTI given from thebase station. Furthermore, the message is used to check whether thetransmission timing is correct. The contention resolution (Message 4) isused to check the collision, and includes meaning of a response toMessage 3. Therefore, Message 3 is not required when the mobile stationis specified up to Message 2, and Message 4 is not required when it isconfirmed that any collision does not occur.

The present invention was made in view of such circumstances, and it isan object of the invention to provide a mobile communication system,base station apparatus and mobile station apparatus enabling the timespent at the time of handover to be shortened when the mobile stationapparatus performs random access for handover.

Means for Solving the Problem

(1) To attain the above-mentioned object, the present invention tookmeasures as described below. In other words, a mobile communicationsystem according to the invention is a mobile communication system inwhich a mobile station apparatus uses any signature of a beforehanddetermined signature group between the mobile station apparatus and abase station apparatus in random access, and is characterized in thatthe base station apparatus selects a signature used by the mobilestation apparatus in random access performed at the time of handover,and the mobile station apparatus performs handover using the signatureselected in the base station apparatus and judges the presence orabsence of transmission and reception of a handover complete message andcontention resolution corresponding to the type of signature selected inthe base station apparatus.

Thus, the mobile station apparatus judges the presence or absence oftransmission and reception of a handover complete message and contentionresolution corresponding to the type of signature selected in the basestation apparatus, and therefore, corresponding to the type ofsignature, it is possible to reduce the time required for transmissionand reception of these messages when the messages are not necessary. Itis thereby possible to shorten the time spent at the time of handover.

(2) In the mobile communication system according to the invention, thebase station apparatus is characterized by determining the presence orabsence of transmission and reception of a handover complete message andcontention resolution corresponding to whether or not the signatureselected in the base station apparatus is a signature for handover.

Thus, the presence or absence of transmission and reception of ahandover complete message and contention resolution is determinedcorresponding to whether or not the signature selected in the basestation apparatus is a signature for handover, and therefore, it ispossible to shorten the time spent at the time of handover while copingwith the collision between mobile station apparatuses when a largenumber of handovers occur at the same time.

(3) Particularly, in the mobile communication system of the invention,the base station apparatus is characterized by determining thattransmission and reception of a handover complete message and contentionresolution is not performed when the signature selected in the basestation apparatus is a signature for handover, while determining thattransmission and reception of a handover complete message and contentionresolution is performed when the signature selected in the base stationapparatus is not a signature for handover.

Thus, it is determined that transmission and reception of a handovercomplete message, etc. is not performed when the signature selected inthe base station apparatus is a signature for handover, and thattransmission and reception of a handover complete message and contentionresolution is performed when the signature selected in the base stationapparatus is not a signature for handover, and therefore, it is possibleto shorten the time spent at the time of handover while coping with thecollision between mobile station apparatuses when a large number ofhandovers occur at the same time.

(4) Further, in the mobile communication system of the invention, thebase station apparatus is characterized by including a flag indicativeof whether or not to perform transmission and reception of a handovercomplete message and contention resolution in a massage to betransmitted to the mobile station apparatus.

Thus, a flag indicative of whether or not to perform transmission andreception of a handover complete message and contention resolution isincluded in a massage to be transmitted to the mobile station apparatus,and the mobile station apparatus is thereby capable of judging whetheror not to perform transmission and reception of a handover completemessage and contention resolution easily based on the flag.

(5) Further, in the mobile communication system of the invention, thebase station apparatus is characterized by determining whether or not toselect a signature for handover as a signature to select correspondingto communication conditions with the mobile station apparatus.

Thus, the base station apparatus determines whether or not to select asignature for handover as a signature to select corresponding tocommunication conditions with the mobile station apparatus, andtherefore, in consideration of communication conditions with the mobilestation apparatus, it is possible to determine whether or not to performtransmission and reception of a handover complete message and contentionresolution.

(6) Further, in the mobile communication system of the invention, thebase station apparatus is characterized by determining whether or not toselect a signature for handover as a signature to select correspondingto Qos of traffic in the mobile station apparatus.

Thus, the base station apparatus determines whether or not to select asignature for handover as a signature to select corresponding to Qos oftraffic in the mobile station apparatus, and therefore, for example, itis possible to shorten the handover time to maintain real-timecharacteristics for a mobile station apparatus that transmits andreceives data with high real-time characteristics. Meanwhile, for amobile station apparatus that transmits and receives data with lowreal-time characteristics, it is possible to perform conventional stablehandover.

(7) A base station apparatus of the invention is a base stationapparatus connected to a mobile station apparatus using any signature ofa beforehand determined signature group in random access, andcharacterized by having a signature managing section that selects asignature used by the mobile station apparatus in random accessperformed at the time of handover, and a message transmissiondetermining section that determines whether or not to performtransmission and reception of a handover complete message and contentionresolution corresponding to the type of signature selected in thesignature managing section.

Thus, whether or not to perform transmission and reception of a handovercomplete message and contention resolution is determined correspondingto the type of signature selected in the signature managing section, andtherefore, corresponding to the type of signature, it is possible toreduce the time required for transmission and reception of thesemessages when the messages are not necessary. It is thereby possible toshorten the time spent at the time of handover.

(8) In the base station apparatus of the invention, the messagetransmission determining section is characterized by determining thattransmission and reception of a handover complete message and contentionresolution is not performed when the signature selected in the signaturemanaging section is a signature for handover, and that transmission andreception of a handover complete message and contention resolution isperformed when the signature selected in the signature managing sectionis not a signature for handover.

Thus, it is determined that transmission and reception of a handovercomplete message, etc. is not performed when the signature selected inthe signature managing section is a signature for handover, and thattransmission and reception of a handover complete message and contentionresolution is performed when the signature selected in the signaturemanaging section is not a signature for handover, and therefore, it ispossible to shorten the time spent at the time of handover while copingwith the collision between mobile station apparatuses when a largenumber of handovers occur at the same time.

(9) In the base station apparatus of the invention, the messagetransmission determining section is characterized by including a flagindicative of whether or not to perform transmission and reception of ahandover complete message and contention resolution in a massage to betransmitted to the mobile station apparatus.

Thus, a flag indicative of whether or not to perform transmission andreception of a handover complete message and contention resolution isincluded in a massage to be transmitted to the mobile station apparatus,and the mobile station apparatus is thereby capable of judging whetheror not to perform transmission and reception of a handover completemessage and contention resolution easily based on the flag.

(10) Further, in the base station apparatus of the invention, thesignature managing section is characterized by determining whether ornot to select a signature for handover as a signature to selectcorresponding to communication conditions with the mobile stationapparatus.

Thus, the signature managing section determines whether or not to selecta signature for handover as a signature to select corresponding tocommunication conditions with the mobile station apparatus, andtherefore, in consideration of communication conditions with the mobilestation apparatus, it is possible to determine whether or not to performtransmission and reception of a handover complete message and contentionresolution.

(11) In the base station apparatus of the invention, the signaturemanaging section is characterized by determining whether or not toselect a signature for handover as a signature to select correspondingto Qos of traffic in the mobile station apparatus.

Thus, the signature managing section determines whether or not to selecta signature for handover as a signature to select corresponding to Qosof traffic in the mobile station apparatus, and therefore, for example,it is possible to shorten the handover time to maintain real-timecharacteristics for a mobile station apparatus that transmits andreceives data with high real-time characteristics. Meanwhile, for amobile station apparatus that transmits and receives data with lowreal-time characteristics, it is possible to perform conventional stablehandover.

(12) A mobile station apparatus of the invention is a mobile stationapparatus using any signature of a beforehand determined signature groupbetween the mobile station apparatus and a base station apparatus inrandom access, and is characterized by having a receiving section thatreceives a message including a flag indicative of whether or not toperform transmission and reception of a handover complete message andcontention resolution from the base station apparatus, and a judgingsection that judges whether or not to perform transmission and receptionof a handover complete message and contention resolution correspondingto content of the flag.

Thus, the judging section judges whether or not to perform transmissionand reception of a handover complete message and contention resolutioncorresponding to the content of a flag included in the message receivedfrom base station apparatus, and it is thereby possible to reduce thetime required for transmission and reception of these messages when themessages are not necessary. It is thus possible to shorten the timespent at the time of handover.

Advantageous Effect of the Invention

According to the invention, the mobile station apparatus judges thepresence or absence of transmission and reception of a handover completemessage and contention resolution corresponding to the type of signatureselected in the base station apparatus, and therefore, corresponding tothe type of signature, it is possible to reduce the time required fortransmission and reception of these messages when the messages are notnecessary. It is thereby possible to shorten the time spent at the timeof handover.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram showing an example of a configuration of abase station apparatus included in a communication system according toEmbodiment 1 of the invention;

FIG. 2 is a block diagram showing an example of a configuration of amobile station apparatus included in the communication system accordingto Embodiment 1;

FIG. 3 is a sequence chart to explain an example of a random accessprocedure at the time of executing handover in the communication systemaccording to Embodiment 1;

FIG. 4 is a sequence chart to explain an example of another randomaccess procedure at the time of executing handover in the communicationsystem according to Embodiment 1;

FIG. 5 is a flowchart to explain operation at the time of executinghandover in the base station apparatus included in the communicationsystem according to Embodiment 1;

FIG. 6 is a flowchart to explain operation at the time of executinghandover in the mobile station apparatus included in the communicationsystem according to Embodiment 1;

FIG. 7 is a block diagram showing an example of a configuration of abase station apparatus included in a communication system according toEmbodiment 2 of the invention;

FIG. 8 is a sequence chart to explain an example of a random accessprocedure at the time of executing handover in the communication systemaccording to Embodiment 2;

FIG. 9 is a sequence chart to explain an example of another randomaccess procedure at the time of executing handover in the communicationsystem according to Embodiment 2;

FIG. 10 is a flowchart to explain operation at the time of executinghandover in the base station apparatus included in the communicationsystem according to Embodiment 2;

FIG. 11 is another flowchart to explain operation at the time ofexecuting handover in the base station apparatus included in thecommunication system according to Embodiment 2;

FIG. 12 is a block diagram showing an example of a configuration of abase station apparatus included in a communication system according toEmbodiment 3 of the invention;

FIG. 13 is a sequence chart to explain an example of a random accessprocedure at the time of executing handover in the communication systemaccording to Embodiment 3;

FIG. 14 is a sequence chart to explain an example of another randomaccess procedure at the time of executing handover in the communicationsystem according to Embodiment 3;

FIG. 15 is a flowchart to explain operation at the time of executinghandover in the base station apparatus included in the communicationsystem according to Embodiment 3;

FIG. 16 is another flowchart to explain operation at the time ofexecuting handover in the base station apparatus included in thecommunication system according to Embodiment 3;

FIG. 17 is a block diagram showing an example of a configuration of abase station apparatus included in a communication system according toEmbodiment 4 of the invention;

FIG. 18 is a sequence chart to explain an example of a random accessprocedure at the time of executing handover in the communication systemaccording to Embodiment 4;

FIG. 19 is a sequence chart to explain an example of another randomaccess procedure at the time of executing handover in the communicationsystem according to Embodiment 4;

FIG. 20 is a flowchart to explain operation at the time of executinghandover in the base station apparatus included in the communicationsystem according to Embodiment 4;

FIG. 21 is a flowchart to explain operation at the time of executinghandover in the mobile station apparatus included in the communicationsystem according to Embodiment 4;

FIG. 22 is a diagram to explain a configuration of uplink in EUTRA;

FIG. 23 is a diagram to explain an uplink random access channel inE-UTRA;

FIG. 24 is a sequence chart to explain an example of a conventionalprocedure of random access;

FIG. 25 is a diagram to explain locations of base stations wherehandover arises; and

FIG. 26 is a sequence chart to explain an example of a conventionalprocedure of random access at the time of executing handover.

DESCRIPTION OF SYMBOLS

-   100 Base station apparatus (Base station)-   101 Data control section-   102 OFDM modulation section-   103 Scheduling section-   104 Radio section-   105 Channel estimation section-   106 DFT-S-OFDM demodulation section-   107 Control data extracting section-   108, 1201, 1701 Preamble detecting section-   109, 1702 Signature managing section-   110, 701, 1202, 1703 Message transmission determining section-   200 Mobile station apparatus (Mobile station)-   201 Data control section-   202 DFT-S-OFDM modulation section-   203 Scheduling section-   204 Signature selecting section-   205 Preamble generating section-   206 Synchronization correcting section-   207 Radio section-   208 Channel estimation section-   209 OFDM demodulation section-   210 Control data extracting section

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments of the invention will specifically be described below withreference to accompanying drawings.

Embodiment 1

In a mobile communication system (hereinafter, referred to as a“communication system” as appropriate) according to Embodiment 1 of theinvention, a base station apparatus (hereinafter, referred to as a “basestation” as appropriate) performs selection of signatures that hasconventionally been performed in a mobile station apparatus(hereinafter, referred to as a “mobile station” as appropriate). Then,by detecting a preamble from a mobile station, the base stationspecifies the mobile station that performs handover, while calculatingsynchronization deviation information, and notifies the mobile stationof the synchronization deviation information.

Thus, in the communication system, since the base station selects asignature for handover of a mobile station, while specifying the mobilestation that performs handover corresponding to a preamble from themobile station, there is no need of a handover complete message (Message3). Further, since the base station selects a signature for handover ofa mobile station, the collision does not occur. Therefore, it ispossible to eliminate the processing after the handover complete message(Message 3), and it is thereby possible to shorten the time spent at thetime of handover.

However, in the communication system, when a large number of handoversoccur at the same time, as a result of lack of signatures for handover,the need arises for selecting a signature for use except handover. Inthis case, the collision may occur with another mobile station thatperforms random access using the signature for use except handover. Inthis case, it is necessary to perform the processing after the handovercomplete message (Message 3) to cope with such a collision.

Therefore, in the communication system, the presence or absence oftransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) is determined corresponding towhether or not a signature selected in the base station is a signaturefor handover. By this means, when these messages are not necessary, itis possible to reduce the time required for transmission and receptionof the messages. Therefore, it is possible to shorten the time spent atthe time of handover while coping with the collision between mobilestations when a large number of handovers occur at the same time.

In addition, in the following description, the signature for handover isa signature which is designated by the base station individually andthereby aims to prevent a collision from occurring between mobilestations. Further, ordinary signatures are signatures which can beselected by mobile stations and have the possibility that the collisionoccurs between mobile stations.

Described below are configurations of the base station and mobilestation included in the communication system according to thisEmbodiment. FIG. 1 is a block diagram showing an example of aconfiguration of the base station included in the communication systemaccording to Embodiment 1. FIG. 2 is a block diagram showing an exampleof a configuration of the mobile station included in a communicationsystem according to Embodiment 1.

As shown in FIG. 1, the base station 100 is comprised of a data controlsection 101, OFDM modulation section 102, scheduling section 103, radiosection 104, channel estimation section 105, DFT-Spread-OFDMdemodulation section (DFT-S-OFDM demodulation section) 106, control dataextracting section 107, preamble detecting section 108, signaturemanaging section 109 and message transmission determining section 110.

The data control section 101 receives inputs of control data and userdata, and corresponding to directions from the scheduling section 103,performs mapping of the control data on a downlink shared controlchannel, downlink synchronized channel, downlink pilot channel anddownlink shared control signaling channel, while further performingmapping of transmission data (user data) to each mobile station on ashared data channel.

The OFDM modulation section 102 performs data modulation,serial/parallel transform of an input signal, IFFT (Inverse Fast FourierTransform), CP (Cyclic Prefix) insertion, filtering and the like, andthus performs OFDM signal processing to generate an OFDM signal.

The scheduling section 103 is comprised of a DL scheduling section 103 athat performs downlink scheduling, and UL scheduling 103 b that performsuplink scheduling. The DL scheduling section 103 a performs schedulingto perform mapping of user data on each downlink channel from CQIinformation notified from the mobile station, and data information ofeach user notified from a higher layer. The UL scheduling section 103 bperforms scheduling to perform mapping of user data on each uplinkchannel from an uplink radio propagation path estimation result from thechannel estimation section 105, and resource allocation request from themobile station.

The radio section 104 up-converts the OFDM modulated data into aradio-frequency signal to transmit to the mobile station. Further, theradio section 104 receives uplink data from the mobile station,down-converts the data to a baseband signal, and outputs the receptiondata to the channel estimation section 105, DFT-S-OFDM demodulationsection 106 and preamble detecting section 108.

The channel estimation section 105 estimates radio propagation pathcharacteristics from the uplink pilot channel UPiCH, and outputs anestimation result to the DFT-S-OFDM demodulation section 106. Further,to perform uplink scheduling, the section 105 outputs the radiopropagation path estimation result to the scheduling section 103. Inaddition, as an uplink communication system, a single-carrier system isassumed such as DFT-S-OFDM and the like, but a multicarrier system isalso allowed such as the OFDM system and the like.

The DFT-S-OFDM demodulation section 106 demodulates the reception datainput from the radio section 104, corresponding to the radio propagationpath estimation result from the channel estimation section 105.

The control data extracting section 107 divides the reception data intouser data (uplink shared data channel USDCH) and control data (uplinkshared control signaling channel USCSCH). Among the divided controldata, the downlink CQI information is output to the scheduling section103, and the other control data and user data is output to the higherlayer.

The preamble detecting section 108 detects a preamble, calculates asynchronization timing deviation amount, and reports the signature IDnumber and synchronization timing deviation amount to the higher layer.When the signature ID number is for handover, the section 108 checkswhether or not the signature ID number is of a signature used by thebase station 100 with the signature managing section 109. As a result ofthe check, when the signature is used by the base station 100, thesection 108 reports the signature ID number and synchronization timingdeviation amount to the higher layer. On the other hand, when thesignature is not used by the base station 100, the section 108 does notreport the signature ID number and synchronization timing deviationamount to the higher layer.

The signature managing section 109 selects a signature corresponding todirections from the higher layer, and notifies the higher layer of theID number (signature ID number) of the selected signature. Further, thesection 109 notifies the selected signature to the preamble detectingsection 108 and message transmission determining section 110. Inaddition, in selecting a signature, the section 109 selectspreferentially from among signatures for handover, and when all suchsignatures are used, selects a signature to use from among signaturesfor uses except handover. At this point, the signature managing section109 confirms signature ID numbers being currently used, and selects onefrom among signatures except signatures being used. Further, thesignature managing section 109 stores the selected signature ID number,and deletes the signature detected in the preamble detecting section 108from the stored content.

The message transmission determining section 110 determines whether ornot to perform transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4) from the signatureinformation from the signature managing section 109, and notifies thehigher layer of the result as flag information.

Meanwhile, as shown in FIG. 2, the mobile station 200 is comprised of adata control section 201, DFT-S-OFDM modulation section 202, schedulingsection 203, signature selecting section 204, preamble generatingsection 205, synchronization correcting section 206, radio section 207,channel estimation section 208, OFDM demodulation section 209 andcontrol data extracting section 210.

The data control section 201 receives inputs of user data and controldata, and corresponding to directions from the scheduling section 203,performs mapping of the data on an uplink scheduling channel.

The DFT-S-OFDM modulation section 202 modulates the data, performsDFT-S-OFDM signal processing such as DFT transform, subcarrier mapping,IFFT, CP (Cyclic Prefix) insertion, filtering and the like, andgenerates a DFT-Spread-OFDM signal. In addition, as an uplinkcommunication system, a single-carrier system is assumed such asDFT-Spread OFDM and the like, but a multicarrier system is also allowedsuch as the OFDM system.

The scheduling section 203 performs scheduling to perform mapping ofuser data on each uplink channel from CQI information notified from thechannel estimation section 208 described later, and schedulinginformation notified from the higher layer.

The signature selecting section 204 selects a signature ID number to usein random access corresponding to directions from the higher layer. Asdirections from the higher layer, the purpose of random access isnotified. When the notified purpose is handover, the section 204 selectsthe signature ID number instructed from the higher layer. Meanwhile,when the notified purpose is not handover, the section 204 randomlyselects a signature from among signatures for uses except handovercorresponding to the purpose, and outputs the selected signature IDnumber to the preamble generating section 205.

The preamble generating section 205 generates a preamble using thesignature ID number selected in the signature selecting section 204 tooutput to the DFT-S-OFDM modulation section 202.

The synchronization correcting section 206 determines transmissiontiming from the synchronization information input from the control dataextracting section 210, and outputs data modulated to adapt to thetransmission timing to the radio section 207.

The radio section 207 up-converts the modulated data into aradio-frequency signal to transmit to the base station 100. Further, theradio section 207 receives downlink data from the base station 100 todown-convert into a baseband signal, and outputs reception data to theOFDM demodulation section 209.

The channel estimation section 208 estimates radio propagation pathcharacteristics from the downlink pilot channel, and outputs theestimation result to the OFDM demodulation section 209. Further, thesection 208 converts the result into CQI information to notify the basestation 100 of the radio propagation path estimation result, and outputsthe CQI information to the scheduling section 203.

The OFDM demodulation section 209 demodulates the reception data inputfrom the radio section 207, corresponding to the radio propagation pathestimation result input from the channel estimation section 208.

The control data extracting section 210 divides the reception data intouser data and control data. The scheduling information in the dividedcontrol data is output to the scheduling section 203, uplinksynchronization information is output to the synchronization correctingsection 206, and the other control data and user data is output to thehigher layer. In addition, the radio section 207, OFDM demodulationsection 209 and control data extracting section 210 constitute thereceiving section, and the scheduling section 203 constitutes thejudging section.

Described next is an example of a random access procedure at the time ofexecuting handover in the communication system according toEmbodiment 1. FIGS. 3 and 4 are sequence charts to explain an example ofa random access procedure at the time of executing handover in thecommunication system according to Embodiment 1. FIG. 3 shows the case ofnot performing transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4), and FIG. 4 shows thecase of performing transmission and reception of these messages. Inaddition, it is assumed herein that the mobile station 200 is currentlyheld by a base station 100A (hereinafter, referred to as a “base stationA” as appropriate). Further, in FIG. 4, the same processing as shown inFIG. 3 is assigned the same symbol to omit descriptions thereof.

As shown in FIGS. 3 and 4, in the random access procedure at the time ofexecuting handover in the communication system according to thisEmbodiment, as a preparatory stage of handover, the mobile station 200first measures radio signal conditions of adjacent base stations(ST301). Then, the mobile station 200 transmits the measurement result(measurement report) to the base station A that is the local-basestation (ST302).

Upon receiving the measurement result from the mobile station 200, thebase station A selects a base station optimal as a handover destinationfrom the measurement result (ST303). In addition, herein, as the optimalbase station, a base station 100B (hereinafter, referred to as a “basestation B” as appropriate) is assumed to be selected. Then, the basestation A transmits a handover request message to the base station Bthat is a handover destination (ST304).

Upon receiving the handover request message from the base station A, thebase station B selects one signature from among signatures for handover(ST305). In this case, in order to avoid the collision between mobilestations 200 to perform handover at the time of random access, the basestation B selects a signature from among signatures except signaturesused in the base station B. In addition, when all the signatures forhandover are being used and cannot be used, the base station B selects asignature from among signatures for uses except handover.

Further, based on the signature to use, the base station B judgeswhether transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4) is required (ST306).In addition, in this judgment, in the case of a signature for handover,since the collision does not occur, the base station B judges thattransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) is not performed. Meanwhile, inthe case of a signature for use except handover, since there is apossibility that the collision occurs, the base station B judges thattransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) is performed.

Then, after assigning C-RNTI to the mobile station 200 to performhandover, the base station B transmits a handover request acknowledgemessage including the signature ID number, flag (hereinafter, referredto as a “message necessity flag” or “flag” as appropriate) indicative ofwhether or not Message 3 and Message 4 are required, and C-RNTI to thebase station A as a response to the handover request message (ST307).

Upon receiving the handover request acknowledge message from the basestation B, the base station A transmits a handover command messageincluding the signature ID number, message necessity flag and C-RNTI tothe mobile station 200 (ST308).

Upon receiving the handover command message from the base station A, themobile station 200 checks the message necessity flag, acquires downlinksynchronization with the base station B, and checks a position of therandom access channel from the broadcast channel (ST309). Afteracquiring downlink synchronization, the mobile station 200 selects thesignature ID number added to the handover command message, and transmitsa preamble (random access preamble: Message 1) including the signatureID number to the base station B on the random access channel (ST310).

The base station B detects the signature from the preamble received fromthe mobile station 200. The processing differs corresponding to the typeof signature. When the base station B confirms that the signature is asignature for handover, as shown in FIG. 3, the base station Bcalculates a synchronization timing deviation amount (ST311). Then, thebase station B transmits a preamble response (Message 2) including thesynchronization timing deviation information (synchronizationinformation) and C-RNTI to the mobile station 200 (ST312). In addition,when there is data for the mobile station to transmit on uplink, thebase station B performs uplink scheduling, and includes also thescheduling information in the preamble response (Message 2) to transmitto the mobile station 200.

Upon receiving these pieces of information from the base station B, themobile station 200 recognizes the information as data to the mobilestation 200, and corrects the synchronization timing deviation from thesynchronization information (ST313). Thereafter, the mobile station 200completes the handover.

Meanwhile, when the base station B confirms that the signature is asignature for use except handover, as shown in FIG. 4, the base stationB calculates a synchronization timing deviation amount, and performsscheduling for transmitting a handover complete message (ST401). Then,the base station B transmits a preamble message (Message 2) includingthe synchronization information, scheduling information, signature IDnumber, and temporary C-RNTI to the mobile station 200 (ST402).

Upon receiving these pieces of information from the base station B, themobile station 200 recognizes the information as data to the mobilestation 200, and corrects the synchronization timing deviation from thesynchronization information (ST403). Then, the mobile station 200generates a handover complete message including C-RNTI, and transmitsthe handover complete message (Message 3) with resources subjected toscheduling to the base station B (ST404). In this case, since a messagenecessity flag is set at “0”, the mobile station 200 waits for acontention resolution (Message 4).

Upon receiving the handover complete message (Message 3), the basestation 100 generates a contention resolution (Message 4) to transmit tothe mobile station 200 (ST405). Thereafter, the base station 100completes the handover. Meanwhile, the mobile station 200 receives thecontention resolution (Message 4) and completes the handover.

Described herein are operations at the time of executing handover in thebase station 100 and mobile station 200 included in the communicationsystem according to Embodiment 1. FIG. 5 is a flowchart to explain theoperation at the time of executing handover in the base station 100included in the communication system according to Embodiment 1. FIG. 6is a flowchart to explain the operation at the time of executinghandover in the mobile station 200 included in the communication systemaccording to this Embodiment. In addition, it is assumed that FIG. 5shows the operation of the base station B (base station that is ahandover destination) as shown in FIGS. 3 and 4, and that FIG. 6 showsthe operation of the mobile station 200 as shown in FIGS. 3 and 4.

As shown in FIG. 5, upon receiving a handover request message from thebase station A (ST501), the base station B assigns C-RNTI of the mobilestation 200, while selecting a signature ID number to be used by themobile station 200 (ST502, ST503). Then, the base station B judgeswhether the selected signature is a signature for handover (ST504).

When the selected signature is a signature for handover, the basestation B sets a message necessity flag at “1” indicative of no need oftransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) (ST505). Meanwhile, when thesignature is not a signature for handover, the base station B sets amessage necessity flag at “0” indicative of need of transmission andreception of a handover complete message (Message 3) and contentionresolution (Message 4) (ST506).

Then, the base station B generates a handover request acknowledgemessage including the signature ID number, C-RNTI and message necessityflag as a response to the handover request message (ST507), andtransmits this handover request acknowledge message to the base stationA (ST508).

After transmitting the handover request acknowledge message, the basestation B waits for a preamble (Message 1) to be transmitted from themobile station 200. Then, upon receiving a preamble transmitted from themobile station 200 (ST509), the base station B judges whether asignature included in the preamble is a signature for handover (ST510).

Herein, when the signature is a signature for handover, the base stationB calculates a synchronization timing deviation amount, and generates apreamble response (Message 2) including the synchronization informationand C-RNTI (ST511). Then, the base station B transmits this preambleresponse to the mobile station 200 (ST512).

Meanwhile, when the signature is not a signature for handover in ST502,the base station B calculates a synchronization timing deviation amount,while performing scheduling for transmitting a handover complete message(Message 3) (ST513). Then, the base station B generates a preambleresponse (Message 2) including temporary C-RNTI, the synchronizationinformation and scheduling information (ST514). Then, the base station Btransmits this preamble response to the mobile station 200 (ST515).

After transmitting the preamble response, the base station B waits for ahandover complete message (Message 3) to be transmitted from the mobilestation 200. Then, upon receiving a handover complete messagetransmitted from the mobile station 200 (ST516), the base station Btransmits a contention resolution (Message 4) including C-RNTI to themobile station 200 (ST517). In this way, a series of operation at thetime of executing handover is finished in the base station 100.

In addition, with respect to the judgment and setting (ST504˜ST506) ofthe message necessity flag made in the base station, instead ofperforming in the base station, the mobile station may make a necessityjudgment on transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4) by determining whethera signature included in a notified handover request acknowledge messageis a signature for handover or ordinary signature.

Meanwhile, as shown in FIG. 6, upon receiving a handover command messagefrom the base station A (ST601), the mobile station 200 acquiresdownlink synchronization with the base station B (ST602). Afteracquiring downlink synchronization, the mobile station 200 selects asignature of the signature ID number included in the handover commandmessage (ST603). In other words, the mobile station 200 selects asignature designated by the base station B. Then, the mobile station 200transmits a preamble (Message 1) including the selected signature to thebase station B (ST604).

After transmitting the preamble, the mobile station 200 waits for apreamble response (Message 2) to be transmitted from the base station B.Then, upon receiving a preamble response transmitted from the basestation B (ST605), the mobile station 200 corrects a synchronizationtiming deviation (ST606), and judges whether a message necessity flagincluded in the handover command message is “1” or “0” (ST607).

Herein, when the message necessity flag is “1”, the mobile station 200finishes the processing without any other processing. Meanwhile, whenthe message necessity flag is “0”, the mobile station 200 generates ahandover complete message (Message 3) including C-RNTI (ST608), andtransmits the handover complete message (Message 3) (ST609). When thehandover complete message is transmitted, the base station B transmits acontention resolution (Message 4), and the mobile station 200 receivesthe contention resolution (ST610). In this way, a series of operation atthe time of executing handover is finished in the mobile station 200.

Thus, in the communication system according to Embodiment 1, thepresence or absence of transmission and reception of a handover completemessage (Message 3) and contention resolution (Message 4) is determinedcorresponding to whether or not the signature selected in the basestation 100 is a signature for handover. By this means, it is possibleto reduce the time required for transmission and reception of thesemessages when the messages are not necessary, and it is thereby possibleto shorten the time spent at the time of handover while coping with thecollision between mobile stations when a large number of handovers occurat the same time.

Particularly, in the communication system according to Embodiment 1, amessage to transmit to the mobile station 200 includes a messagenecessity flag indicative of whether or not to perform transmission andreception of a handover complete message (Message 3) and contentionresolution (Message 4), and the mobile station 200 is thereby capable ofjudging whether or not to perform transmission and reception of ahandover complete message (Message 3) and contention resolution (Message4) easily based on the message necessity flag.

Embodiment 2

In the communication system according to Embodiment 1, the presence orabsence of transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4) is determinedcorresponding to whether or not the signature selected in the basestation 100 is a signature for handover. In a communication systemaccording to Embodiment 2, the presence or absence of transmission andreception of a handover complete message (Message 3) and contentionresolution (Message 4) is determined in consideration of Qos of trafficin the mobile station 200 to perform handover, and in this respect, thecommunication system differs from the communication system according toEmbodiment 1.

Described below are configurations of a base station and mobile stationincluded in the communication system according to Embodiment 2. FIG. 7is a block diagram showing an example of a configuration of the basestation 700 included in the communication system according to Embodiment2. In addition, in the base station 700 as shown in FIG. 7, the samecomponents as in FIG. 1 are assigned the same symbols to omitdescriptions thereof. Further, the mobile station included in thecommunication system according to Embodiment 2 has the sameconfiguration as in the mobile station 200 according to Embodiment 1shown in FIG. 2, and descriptions thereof are omitted.

The base station 700 as shown in FIG. 7 has a message transmissiondetermining section 701, and in this respect, differs from the basestation 100 according to Embodiment 1. In the base station 700 accordingto Embodiment 2, the message transmission determining section 701determines whether or not to perform transmission and reception of ahandover complete message (Message 3) and contention resolution (Message4) from the Qos information of the mobile station 200 to performhandover from the higher layer, and notifies the higher layer of theresult as flag information.

Described next is an example of a random access procedure at the time ofexecuting handover in the communication system according to Embodiment2. FIGS. 8 and 9 are sequence charts to explain an example of a randomaccess procedure at the time of executing handover in the communicationsystem according to Embodiment 2. FIG. 8 shows the case of notperforming transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4), and FIG. 9 shows thecase of performing transmission and reception of these messages. Inaddition, it is assumed herein that the mobile station 200 is currentlyheld by the base station 700A (hereinafter, referred to as a “basestation A” as appropriate). Further, in FIGS. 8 and 9, the sameprocessing as shown in FIGS. 3 and 4 is assigned the same symbol to omitdescriptions thereof.

The random access procedure at the time of executing handover in thecommunication system according to Embodiment 2 differs from theprocedure in the communication system according to Embodiment 1principally in the processing for judging whether or not to performtransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4).

In the communication system according to Embodiment 2, whether or not toperform transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4) is judged from the Qosinformation of traffic of the mobile station 200 to perform handover(ST801). In addition, in this judgment, it is judged that transmissionand reception of a handover complete message (Message 3) and contentionresolution (Message 4) is not performed in traffic with high real-timecharacteristics in consideration of the real-time characteristics, andthat transmission and reception of a handover complete message (Message3) and contention resolution (Message 4) is performed in traffic withlow real-time characteristics to guarantee stability of communication.

Further, in the communication system according to Embodiment 1, theprocessing is varied corresponding to the type of signature included ina preamble transmitted from the mobile station 200. In contrast thereto,in the communication system according to Embodiment 2, the processing isvaried corresponding to necessity of transmission and reception of ahandover complete message (Message 3) and contention resolution (Message4) judged in ST801, and in this respect, the communication systemdiffers from the communication system according to Embodiment 1.

In other words, when the message necessity flag indicates “1”, as shownin FIG. 8, a handover complete message (Message 3) and contentionresolution (Message 4) are neither transmitted nor received. Meanwhile,when the message necessity flag indicates “0”, as shown in FIG. 9, ahandover complete message (Message 3) and contention resolution (Message4) are transmitted and received.

Described herein is the operation at the time of executing handover inthe base station 700 included in the communication system according toEmbodiment 2. FIGS. 10 and 11 are flowcharts to explain the operation atthe time of executing handover in the base station 700 included in thecommunication system according to Embodiment 2. In addition, it isassumed that FIGS. 10 and 11 show the operation of the base station B(base station that is a handover destination) as shown in FIGS. 8 and 9.Meanwhile, the operation at the time of executing handover in the mobilestation 200 is the same as the operation as shown in FIG. 6, anddescriptions thereof are omitted.

As shown in FIG. 10, upon receiving a handover request message from thebase station A (ST1001), the base station B assigns C-RNTI of the mobilestation 200, while selecting a signature ID number to be used by themobile station 200 (ST1002, ST1003). Then, the base station B judges theQos information of the mobile station 200 to perform handover (ST1004).More specifically, the base station B judges whether the mobile station200 to perform handover transmits and receives data with high real-timecharacteristics or transmits and receives data with low real-timecharacteristics.

When the mobile station 200 transmits and receives data with highreal-time characteristics, the base station B sets a message necessityflag at “1” indicative of no need of transmission and reception of ahandover complete message (Message 3) and contention resolution (Message4) (ST1005). Meanwhile, when the mobile station 200 transmits andreceives data with low real-time characteristics, the base station Bsets a message necessity flag at “0” indicative of need of transmissionand reception of a handover complete message (Message 3) and contentionresolution (Message 4) (ST1006).

Then, the base station B generates a handover request acknowledgemessage including the signature ID number, C-RNTI and message necessityflag as a response to the handover request message (ST1007), andtransmits this handover request acknowledge message to the base stationA (ST1008).

After transmitting the handover request acknowledge message, the basestation B waits for a preamble (Message 1) to be transmitted from themobile station 200. Then, as shown in FIG. 11, upon receiving a preambletransmitted from the mobile station 200 (ST1101), the base station Bjudges whether a signature included in the preamble is a signature forhandover (ST1102).

Herein, when the signature is not a signature for handover, the basestation B calculates a synchronization timing deviation amount, whileperforming scheduling for transmitting a handover complete message(Message 3) (ST1103). Then, the base station B generates a preambleresponse (Message 2) including temporary C-RNTI, the synchronizationinformation and scheduling information (ST1104). Then, the base stationB transmits this preamble response to the mobile station 200 (ST1105).

After transmitting the preamble response, the base station B waits for ahandover complete message (Message 3) to be transmitted from the mobilestation 200. Then, upon receiving a handover complete messagetransmitted from the mobile station 200 (ST1106), the base station Btransmits a contention resolution (Message 4) including C-RNTI to themobile station 200 (ST1107).

Meanwhile, in ST1102, when the signature is a signature for handover,the base station B judges the flag on whether or not to performtransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) i.e. judges whether the messagenecessity flag is “0” or “1” (ST1108). When the message necessity flagis “1”, the base station B calculates a synchronization timing deviationamount, while performing scheduling for transmitting a handover completemessage (Message 3) (ST1109). Then, the base station B generates apreamble response (Message 2) including temporary C-RNTI, thesynchronization information and scheduling information (ST1110). Then,the base station B transmits this preamble response to the mobilestation 200 (ST1111).

After transmitting the preamble response, the base station B waits for ahandover complete message (Message 3) to be transmitted from the mobilestation 200. Then, upon receiving a handover complete messagetransmitted from the mobile station 200 (ST1112), the base station Btransmits a contention resolution (Message 4) including C-RNTI to themobile station 200 (ST1113).

Meanwhile, in ST1108, when the message necessity flag is “1”, the basestation B judges whether uplink data transmission to the base station700 is performed in the mobile station 200 (ST1114). Herein, when uplinkdata transmission is not performed, the base station B calculates asynchronization timing deviation amount, and generates a preambleresponse (Message 2) including the synchronization information andC-RNTI (ST1115). Then, the base station B transmits this preambleresponse to the mobile station 200 (ST1116).

Meanwhile, when uplink data transmission is performed, the base stationB calculates a synchronization timing deviation amount, and performsscheduling for transmitting data (ST1117). Then, the base station Bgenerates a preamble response (Message 2) including C-RNTI, thesynchronization information and scheduling information (ST1118). Then,the base station B transmits this preamble response to the mobilestation 200 (ST1119). In this way, a series of operation at the time ofexecuting handover is finished in the base station 700.

Thus, in the communication system according to Embodiment 2, whether ornot to perform transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4) is judged using Qosinformation, and therefore, it is possible to shorten the handover timeto maintain real-time characteristics for the mobile station 200 thattransmits and receives data with high real-time characteristics.Meanwhile, for the mobile station 200 that transmits and receives datawith low real-time characteristics, it is possible to performconventional stable handover.

Embodiment 3

In the communication system according to Embodiment 1, the presence orabsence of transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4) is determinedcorresponding to whether or not the signature selected in the basestation 100 is a signature for handover. In a communication systemaccording to Embodiment 3, a handover-destination base station judgesreception conditions of a preamble in receiving the preamble, anddetermines the presence or absence of transmission and reception of ahandover complete message (Message 3) and contention resolution (Message4), and in this respect, the communication system differs from thecommunication system according to Embodiment 1.

Described below are configurations of a base station and mobile stationincluded in the communication system according to Embodiment 3. FIG. 12is a block diagram showing an example of a configuration of the basestation 1200 included in the communication system according toEmbodiment 3. In addition, in the base station 1200 as shown in FIG. 12,the same components as in FIG. 1 are assigned the same symbols to omitdescriptions thereof. Further, the mobile station included in thecommunication system according to Embodiment 3 has the sameconfiguration as in the mobile station 200 according to Embodiment 1shown in FIG. 2, and descriptions thereof are omitted.

The base station 1200 as shown in FIG. 12 has a preamble detectingsection 1201 and message transmission determining section 1202, and inthis respect, differs from the base station 100 according toEmbodiment 1. In the base station 1200 according to Embodiment 3, thepreamble detecting section 1201 detects a preamble from the mobilestation 200, calculates a synchronization timing deviation amount, andreports a signature ID number and synchronization timing deviationamount to the higher layer. Herein, when the signature ID number is forhandover, the preamble detecting section 1201 checks whether or not thesignature ID number is of a signature used by the base station 1200 withthe signature managing section 109. As a result of the check, when thesignature is used by the base station 1200, the preamble detectingsection 1201 reports the signature ID number and synchronization timingdeviation amount to the higher layer. Meanwhile, when the signature isnot used by the base station 1200, the preamble detecting section 1201does not report the signature ID number and synchronization timingdeviation amount to the higher layer. Further, when the signature is forhandover, the preamble detecting section 1201 outputs a correlationresult to the message transmission determining section 1202. The messagetransmission determining section 1202 determines whether or not toperform transmission and reception of a handover complete message(Message 3) and subsequent message from the correlation result from thepreamble detecting section 1201, and notifies the higher layer of theresult as flag information.

Described next is an example of a random access procedure at the time ofexecuting handover in the communication system according to Embodiment3. FIGS. 13 and 14 are sequence charts to explain an example of a randomaccess procedure at the time of executing handover in the communicationsystem according to Embodiment 3. FIG. 13 shows the case of notperforming transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4), and FIG. 14 shows thecase of performing transmission and reception of these messages. Inaddition, it is assumed herein that the mobile station 200 is currentlyheld by a base station 1200A (hereinafter, referred to as a “basestation A” as appropriate). Further, in FIG. 14, the same processing asshown in FIG. 13 is assigned the same symbol to omit descriptionsthereof.

As shown in FIGS. 13 and 14, in the random access procedure at the timeof executing handover in the communication system according toEmbodiment 3, as a preparatory stage of handover, the mobile station 200first measures radio signal conditions of adjacent base stations(ST1301). Then, the mobile station 200 transmits the measurement result(measurement report) to the base station A that is the local-basestation (ST1302).

Upon receiving the measurement result from the mobile station 200, thebase station A selects a base station optimal as a handover destinationfrom the measurement result (ST1303). In addition, herein, as theoptimal base station, a base station 1200B (hereinafter, referred to asa “base station B” as appropriate) is assumed to be selected. Then, thebase station A transmits a handover request message to the base stationB that is a handover destination (ST1304).

Upon receiving the handover request message from the base station A, thebase station B selects one signature from among signatures for handover(ST1305). In this case, in order to avoid the collision between mobilestations to perform handover at the time of random access, the basestation B selects a signature from among signatures except signaturesused in the base station B.

Then, after assigning C-RNTI to the mobile station 200 to performhandover, the base station B transmits a handover request acknowledgemessage including the signature ID number and C-RNTI to the base stationA as a response to the handover request message (ST1306).

Upon receiving the handover request acknowledge message from the basestation B, the base station A transmits a handover command messageincluding the signature ID number and C-RNTI to the mobile station 200(ST1307).

Upon receiving the handover command message from the base station A, themobile station 200 acquires downlink synchronization with the basestation B, and checks a position of the random access channel from thebroadcast channel (ST1308). After acquiring downlink synchronization,the mobile station 200 selects the signature ID number added to thehandover command message, and transmits a preamble (random accesspreamble: Message 1) including the signature ID number to the basestation B on the random access channel (ST1309).

The base station B detects the signature from the preamble received fromthe mobile station 200. When the base station B confirms that thesignature is a signature for handover, the base station B calculates asynchronization timing deviation amount (ST1310). Then, from receptionconditions of the preamble, the base station B judges whethertransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) is necessary (ST1311).

In this judgment, when correlation characteristics of the preamble arehigher than a threshold, the base station B judges that propagation pathconditions are good, and that transmission and reception of a handovercomplete message (Message 3) and contention resolution (Message 4) isnot performed. Alternately, when correlation characteristics of thepreamble are lower than the threshold, the base station B judges thatpropagation path conditions are poor, and that transmission andreception of a handover complete message (Message 3) and contentionresolution (Message 4) is performed to obtain reliability.

Herein, when it is determined that transmission and reception of ahandover complete message (Message 3) and contention resolution (Message4) is not necessary, the base station B transmits a preamble response(Message 2) including the synchronization timing deviation information(synchronization information), C-RNTI and message necessity to themobile station 200 (ST1312).

Upon receiving these pieces of information from the base station B, themobile station 200 recognizes the information as data to the mobilestation 200, corrects the synchronization timing deviation from thesynchronization information (ST1313), checks the message necessity flag,and waits for data from the base station 1200. In addition, when thereis data for the mobile station 200 to transmit on uplink, the basestation 1200 performs uplink scheduling, and includes also thescheduling information in the preamble response (Message 2) to transmitto the mobile station 200. In this case, upon receiving the data to thestation 200, the mobile station 200 corrects the synchronization timingdeviation from the synchronization information, checks the messagenecessity flag and starts transmission of the data of the mobile station200 with resources subjected to scheduling.

Meanwhile, when the base station B judges that transmission andreception of a handover complete message (Message 3) and contentionresolution (Message 4) is required, the base station B performsscheduling for transmitting a handover complete message, and as shown inFIG. 14, transmits a preamble response (Message 2) including thesynchronization information, scheduling information, message necessityflag and C-RNTI to the mobile station 200 (ST1401). In addition, in thiscase, since the mobile station 200 is beforehand notified of C-RNTI, thesignature ID number is not necessary as the identification informationfor the mobile station 200 to identify the data to the station 200, andC-RNTI is only required.

Upon receiving the data to the mobile station 200, the station 200corrects the synchronization timing deviation from the synchronizationinformation (ST1402). Then, the mobile station 200 checks the messagenecessity flag, and transmits a handover complete message includingC-RNTI with resources subjected to scheduling to the base station B(ST1403).

Upon receiving the handover complete message, the base station Bgenerates a contention resolution (Message 4) to transmit to the mobilestation 200 (ST1404). Thereafter, the base station B completes thehandover. Meanwhile, the mobile station 200 receives the contentionresolution (Message 4) and completes the handover.

Described herein is operation at the time of executing handover in thebase station 1200 included in the communication system according toEmbodiment 3. FIGS. 15 and 16 are flowcharts to explain the operation atthe time of executing handover in the base station 1200 included in thecommunication system according to Embodiment 3. In addition, it isassumed that FIGS. 15 and 16 show the operation of the base station B(base station that is a handover destination) as shown in FIGS. 13 and14. The operation at the time of executing handover in the mobilestation 200 is the same as the operation shown in FIG. 6 anddescriptions thereof are omitted.

As shown in FIG. 15, upon receiving a handover request message from thebase station A (ST1501), the base station B assigns C-RNTI of the mobilestation 200, while selecting a signature ID number to be used by themobile station 200 (ST1502, ST1503). Then, the base station B generatesa handover request acknowledge message including the signature ID numberand C-RNTI as a response to the handover request message (ST1504), andtransmits this handover request acknowledge message to the base stationA (ST1505).

After transmitting the handover request acknowledge message, the basestation B waits for a preamble (Message 1) to be transmitted from themobile station 200. Then, as shown in FIG. 16, upon receiving a preambletransmitted from the mobile station 200 (ST1601), the base station Bjudges whether a signature included in the preamble is a signature forhandover (ST1602).

Herein, when the signature is not a signature for handover, the basestation B sets a message necessity flag at “0” (ST1603). Then, the basestation B calculates a synchronization timing deviation amount, whileperforming scheduling for transmitting a handover complete message(Message 3) (ST1604). Then, the base station B generates a preambleresponse (Message 2) including the signature ID number, temporaryC-RNTI, synchronization information, scheduling information and messagenecessity flag (ST1605). Then, the base station B transmits thispreamble response to the mobile station 200 (ST1606).

After transmitting the preamble response, the base station B waits for ahandover complete message (Message 3) to be transmitted from the mobilestation 200. Then, upon receiving a handover complete messagetransmitted from the mobile station 200 (ST1607), the base station Btransmits a contention resolution (Message 4) including C-RNTI to themobile station 200 (ST1608).

Meanwhile, when the signature is a signature for handover, the basestation B judges whether or not to perform transmission and reception ofa handover complete message (Message 3) and contention resolution(Message 4) i.e. whether a correlation value of the preamble is higheror lower than a given value (ST1609) for a flag.

When the correlation value of the preamble is lower than the givenvalue, the base station B sets a message necessity flag at “0” (ST1610).Then, the base station B calculates a synchronization timing deviationamount, while performing scheduling for transmitting a handover completemessage (Message 3) (ST1611). Then, the base station B generates apreamble response (Message 2) including the signature ID number, C-RNTI,synchronization information, scheduling information and messagenecessity flag (ST1612). Then, the base station B transmits thispreamble response to the mobile station 200 (ST1613).

After transmitting the preamble response, the base station B waits for ahandover complete message (Message 3) to be transmitted from the mobilestation 200. Then, upon receiving a handover complete messagetransmitted from the mobile station 200 (ST1614), the base station Btransmits a contention resolution (Message 4) including C-RNTI to themobile station 200 (ST1615).

Meanwhile, when the correlation value of the preamble is higher than thegiven value in ST1609, the base station B judges whether uplink datatransmission to the base station 1200 is performed in the mobile station200 (ST1616). Herein, when uplink data transmission is not performed,the base station B sets a message necessity flag at “1” (ST1617). Then,the base station B calculates a synchronization timing deviation amount,and generates a preamble response (Message 2) including C-RNTI, thesynchronization information and message necessity flag (ST1618). Then,the base station B transmits this preamble response to the mobilestation 200 (ST1619).

Meanwhile, when uplink data transmission is performed, after setting amessage necessity flag at “1” (ST1620), the base station B calculates asynchronization timing deviation amount, and performs scheduling fortransmitting the data (ST1621). Then, the base station B generates apreamble response (Message 2) including C-RNTI, the synchronizationinformation, scheduling information and message necessity flag (ST1622).Then, the base station B transmits this preamble response to the mobilestation 200 (ST1623). Thus, a series of operation at the time ofexecuting handover is finished in the base station 1200.

Thus, in the communication system according to Embodiment 3, the basestation B judges whether or not to perform transmission and reception ofa handover complete message (Message 3) and contention resolution(Message 4) based on propagation path conditions in receiving thepreamble, and it is thereby possible to shorten the connection time forthe mobile station 200 with good propagation path conditions. Meanwhile,it is possible to perform conventional stable handover for the mobilestation 200 with poor propagation path conditions.

Embodiment 4

In a communication system according to Embodiment 4, while Qos oftraffic in the mobile station 200 to perform handover is considered, thepresence or absence of transmission and reception of a handover completemessage (Message 3) and contention resolution (Message 4) is determinedcorresponding to whether or not the signature selected in the basestation is a signature for handover. In other words, this Embodimentcorresponds to an Embodiment obtained by combining the communicationsystems according to Embodiments 1 and 2.

Described below are configurations of a base station and mobile stationincluded in the communication system according to Embodiment 4. FIG. 17is a block diagram showing an example of a configuration of the basestation 1700 included in the communication system according toEmbodiment 4. In addition, in the base station 1700 as shown in FIG. 17,the same components as in FIG. 1 are assigned the same symbols to omitdescriptions thereof. Further, the mobile station included in thecommunication system according to Embodiment 4 has the sameconfiguration as in the mobile station 200 according to Embodiment 1shown in FIG. 2, and descriptions thereof are omitted.

The base station 1700 as shown in FIG. 17 has a preamble detectingsection 1701, signature managing section 1702 and message transmissiondetermining section 1703, and in this respect, differs from the basestation 100 according to Embodiment 1. In the base station 1700according to Embodiment 4, the preamble detecting section 1701 detects apreamble, calculates a synchronization timing deviation amount, andreports the signature ID number and synchronization timing deviationamount to the higher layer. When the signature ID number is a signatureselected in the signature managing section 1702, the preamble detectingsection 1701 notifies the signature managing section 1702 of thesignature ID number.

The signature managing section 1702 receives Qos information from thehigher layer, and selects a signature to notify the higher layer. Thesection 1702 selects a signature from among signatures for handover whenQos of traffic is of high real-time characteristics, while selecting asignature from among signatures for uses except handover when Qos oftraffic is of low real-time characteristics. Further, the section 1702checks signature numbers being used, and selects a signature from amongsignatures except the used signatures. Moreover, the section 1702 storesthe selected signature ID number, and deletes the signature detected inthe preamble detecting section 1701 from the stored content. The section1702 notifies the preamble detecting section 1701 and messagetransmission determining section 1703 of the selected signature IDnumber. The message transmission determining section 1703 determineswhether or not to perform transmission and reception of a handovercomplete message (Message 3) and contention resolution (Message 4) fromthe signature information from the signature managing section 1702, andnotifies the higher layer of the result as flag information.

Described next is an example of a random access procedure at the time ofexecuting handover in the communication system according to Embodiment4. FIGS. 18 and 19 are sequence charts to explain an example of a randomaccess procedure at the time of executing handover in the communicationsystem according to Embodiment 4. FIG. 18 shows the case of notperforming transmission and reception of a handover complete message(Message 3) and contention resolution (Message 4), and FIG. 19 shows thecase of performing transmission and reception of these messages. Inaddition, it is assumed herein that the mobile station 200 is currentlyheld by a base station 100A (hereinafter, referred to as a “base stationA” as appropriate).

As shown in FIGS. 18 and 19, in the random access procedure at the timeof executing handover in the communication system according to thisEmbodiment, as a preparatory stage of handover, the mobile station 200first measures radio signal conditions of adjacent base stations(ST1801). Then, the mobile station 200 transmits the measurement result(measurement report) to the base station A that is the local-basestation (ST1802).

Upon receiving the measurement result from the mobile station 200, thebase station A selects a base station optimal as a handover destinationfrom the measurement result (ST1803). In addition, herein, as theoptimal base station, a base station 1700B (hereinafter, referred to asa “base station B” as appropriate) is assumed to be selected. Then, thebase station A transmits a handover request message including Qosinformation of traffic of the mobile station 200 to the base station Bthat is a handover destination (ST1804).

Upon receiving the handover request message from the base station A, thebase station B checks Qos of traffic of the mobile station 200 toperform handover, and selects one signature from among signatures forhandover when Qos of traffic is of high real-time characteristics.Meanwhile, when Qos of traffic is of low real-time characteristics, thebase station B selects one signature from among ordinary signatures(ST1805). In this case, in order to avoid the collision between mobilestations 200 to perform handover at the time of random access, the basestation B selects a signature from among signatures except signaturesused in the base station B.

Further, based on the signature to use, the base station B judgeswhether or not to perform transmission and reception of a handovercomplete message (Message 3) and contention resolution (Message 4)(ST1806). In addition, in this judgment, in the case of a signature forhandover, since the collision does not occur, the base station B judgesthat transmission and reception of a handover complete message (Message3) and contention resolution (Message 4) is not performed. Meanwhile, inthe case of a signature for use except handover, since there is apossibility that the collision occurs, the base station B judges thattransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) is performed.

Then, after assigning C-RNTI to the mobile station 200 to performhandover, the base station B transmits a handover request acknowledgemessage including the signature ID number, message necessity flag andC-RNTI to the base station A as a response to the handover requestmessage (ST1807).

Upon receiving the handover request acknowledge message from the basestation B, the base station A transmits a handover command messageincluding the signature ID number, message necessity flag and C-RNTI tothe mobile station 200 (ST1808).

Upon receiving the handover command message from the base station A, themobile station 200 checks the message necessity flag, acquires downlinksynchronization with the base station B, and checks a position of therandom access channel from the broadcast channel (ST1809). Afteracquiring downlink synchronization, the mobile station 200 selects thesignature ID number added to the handover command message, and transmitsa preamble (random access preamble: Message 1) including the signatureID number to the base station B on the random access channel (ST1810).

The base station B detects the signature from the preamble received fromthe mobile station 200. The processing differs corresponding to the typeof signature. When the base station B confirms that the signature is asignature for handover, as shown in FIG. 18, the base station Bcalculates a synchronization timing deviation amount (ST1811). Then, thebase station B transmits a preamble response (Message 2) including thesynchronization timing deviation information (synchronizationinformation) and C-RNTI to the mobile station 200 (ST1812). In addition,when there is data for the mobile station to transmit on uplink, thebase station B performs uplink scheduling, and includes also thescheduling information in the preamble response (Message 2) to transmitto the mobile station 200.

Upon receiving these pieces of information from the base station B, themobile station 200 recognizes the information as data to the mobilestation 200, and corrects the synchronization timing deviation from thesynchronization information (ST1813). Thereafter, the mobile station 200completes the handover.

Meanwhile, when the base station B confirms that the signature is asignature for use except handover designated by the base station 1700,as shown in FIG. 19, the base station B calculates a synchronizationtiming deviation amount, and performs scheduling for transmitting ahandover complete message (ST1901). Then, the base station B transmits apreamble message (Message 2) including the synchronization information,scheduling information, signature ID number, and temporary C-RNTI to themobile station 200 (ST1902).

Upon receiving these pieces of information from the base station B, themobile station 200 recognizes the information as data to the mobilestation 200, and corrects the synchronization timing deviation from thesynchronization information (ST1903). Then, the mobile station 200generates a handover complete message including C-RNTI, and transmitsthe handover complete message (Message 3) with resources subjected toscheduling to the base station B (ST1904).

Upon receiving the handover complete message (Message 3), the basestation B generates a contention resolution (Message 4) to transmit tothe mobile station 200 (ST1905). Thereafter, the base station Bcompletes the handover. Meanwhile, the mobile station 200 receives thecontention resolution (Message 4) and completes the handover.

Described herein are operations at the time of executing handover in thebase station 1700 and mobile station 200 included in the communicationsystem according to Embodiment 4. FIG. 20 is a flowchart to explain theoperation at the time of executing handover in the base station 1700included in the communication system according to Embodiment 4. FIG. 21is a flowchart to explain the operation at the time of executinghandover in the mobile station 200 included in the communication systemaccording to Embodiment 4. In addition, it is assumed that FIG. 20 showsthe operation of the base station B (base station that is a handoverdestination) as shown in FIGS. 18 and 19, and that FIG. 21 shows theoperation of the mobile station 200 as shown in FIGS. 18 and 19.

As shown in FIG. 20, upon receiving a handover request message from thebase station A (ST2001), the base station B assigns C-RNTI of the mobilestation 200 (ST2002). Then, the base station B judges Qos information ofthe mobile station 200 to perform handover (ST2003). More specifically,the base station B judges whether the mobile station 200 to performhandover transmits and receives data with high real-time characteristicsor transmits and receives data with low real-time characteristics.

Herein, when the mobile station 200 transmits and receives data withhigh real-time characteristics, the base station B selects a signatureID number from among signatures for handover (ST2004). Meanwhile, whenthe mobile station 200 transmits and receives data with low real-timecharacteristics, the base station B selects a signature ID number fromamong ordinary signatures (ST2005). Then, the base station B judgeswhether the selected signature is a signature for handover (ST2006).

When the signature is a signature for handover, the base station B setsa message necessity flag at “1” indicative of no need of transmissionand reception of a handover complete message (Message 3) and contentionresolution (Message 4) (ST2007). Meanwhile, when the signature is not asignature for handover, the base station B sets a message necessity flagat “0” indicative of need of transmission and reception of a handovercomplete message (Message 3) and contention resolution (Message 4)(ST2008).

Then, the base station B generates a handover request acknowledgemessage including the signature ID number, C-RNTI and message necessityflag as a response to the handover request message (ST2009), andtransmits this handover request acknowledge message to the base stationA (ST2010).

After transmitting the handover request acknowledge message, the basestation B waits for a preamble (Message 1) to be transmitted from themobile station 200. Then, upon receiving a preamble transmitted from themobile station 200 (ST2011), the base station B judges whether asignature included in the preamble is a signature for handover (ST2012).

Herein, when the signature is a signature for handover, the base stationB calculates a synchronization timing deviation amount, and generates apreamble response (Message 2) including the synchronization informationand C-RNTI (ST2013). Then, the base station B transmits this preambleresponse to the mobile station 200 (ST2014).

Meanwhile, when the signature is not a signature for handover in ST2012,the base station B calculates a synchronization timing deviation amount,while performing scheduling for transmitting a handover complete message(Message 3) (ST2015). Then, the base station B generates a preambleresponse (Message 2) including temporary C-RNTI, the synchronizationinformation and scheduling information (ST2016). Then, the base stationB transmits this preamble response to the mobile station 200 (ST2017).

After transmitting the preamble response, the base station B waits for ahandover complete message (Message 3) to be transmitted from the mobilestation 200. Then, upon receiving a handover complete messagetransmitted from the mobile station 200 (ST2018), the base station Btransmits a contention resolution (Message 4) including C-RNTI to themobile station 200 (ST2019). In this way, a series of operation at thetime of executing handover is finished in the base station 1700.

Meanwhile, as shown in FIG. 21, upon receiving a handover commandmessage from the base station A (ST2101), the mobile station 200acquires downlink synchronization with the base station B (ST2102).After acquiring downlink synchronization, the mobile station 200 selectsa signature of the signature ID number included in the handover commandmessage (ST2103). In other words, the mobile station 200 selects asignature designated by the base station B. Then, the mobile station 200transmits a preamble (Message 1) including the selected signature to thebase station B (ST2104).

Upon transmitting the preamble, the mobile station 200 waits for apreamble response (Message 2) to be transmitted from the base station B.Then, upon receiving a preamble message transmitted from the basestation B (ST2105), the mobile station 200 corrects a synchronizationtiming deviation (ST2106), and judges whether a message necessity flagincluded in the handover command message is “1” or “0” (ST2107).

Herein, when the message necessity flag is “1”, the mobile station 200finishes the processing without any other processing. Meanwhile, whenthe message necessity flag is “0”, the mobile station 200 generates ahandover complete message (Message 3) including C-RNTI (ST2108). Then,the mobile station 200 transmits the handover complete message (Message3) to the base station B (ST2109). When the handover complete message istransmitted, the base station B transmits a contention resolution(Message 4), and the mobile station 200 receives the contentionresolution (ST2110). In this way, a series of operation at the time ofexecuting handover is finished in the mobile station 200.

In addition, with respect to the judgment and setting (ST2006˜ST2008) ofa message necessity flag made in the base station, instead of performingin the base station, the mobile station may make a necessity judgment ontransmission and reception of a handover complete message (Message 3)and contention resolution (Message 4) by determining whether a signatureincluded in a notified handover request acknowledge message is asignature for handover or ordinary signature.

Thus, according to the communication system according to Embodiment 4,while Qos of traffic in the mobile station 200 to perform handover isconsidered, the presence or absence of transmission and reception of ahandover complete message (Message 3) and contention resolution (Message4) is determined corresponding to whether or not the signature selectedin the base station is a signature for handover. By this means, it ispossible to reduce the time required for transmission and reception ofthese messages when the messages are not necessary, and it is therebypossible to shorten the time spent at the time of handover while copingwith the collision between mobile stations when a large number ofhandovers occur at the same time. Further, it is possible to shorten thehandover time to maintain real-time characteristics for the mobilestation 200 that transmits and receives data with high real-timecharacteristics. Meanwhile, for the mobile station 200 that transmitsand receives data with low real-time characteristics, it is possible toperform conventional stable handover.

The present invention is not limited to the above-mentioned Embodiments,and is capable of being carried into practice with various modificationsthereof. In the above-mentioned Embodiments, sizes, shapes and the likeas shown in the accompanying drawings are not limited thereto, and arecapable of being modified as appropriate within the scope of exhibitingthe effects of the invention. Moreover, the invention is capable ofbeing carried into practice with modifications thereof as appropriatewithout departing from the scope of the object of the invention.

1. A method for random access to a first base station at a time ofhandover, the first base station being configured with a first signaturegroup and a second signature group, the method comprising: receiving, bya mobile station, a message including a reference indicative of randomaccess from a second base station; determining, by the mobile station,whether the reference indicates a signature within the first signaturegroup or the second signature group; sending, by the mobile station, arandom access preamble including the signature indicated by thereference to the first base station in instances where the referenceindicates a signature within the first signature group; and performing,by the mobile station, a contention resolution processing in instanceswhere the reference indicates a signature within the second signaturegroup.
 2. The method according to claim 1, wherein the reference isdetermined by the first base station and transmitted to the second basestation.
 3. The method according to claim 1, wherein contentionresolution processing comprises: receiving, by the mobile station, acontention resolution message from the first base station.
 4. The methodaccording to claim 3, wherein contention resolution processing furthercomprises: randomly selecting, by the mobile station, a signature withinthe second signature group; and sending, by the mobile station, a randomaccess preamble including the randomly selected signature to the firstbase station.
 5. The method according to claim 3, wherein the mobilestation does not receive the contention resolution message from thefirst base station in instances where the reference indicates asignature within the first signature group.
 6. A method for randomaccess to a first base station at a time of handover, the first basestation being configured with a first signature group and a secondsignature group, the method comprising: determining, by the first basestation, a reference used for random access, wherein the referenceindicates a signature within the first signature group or the secondsignature group; sending, by the first base station, a message includingat least a portion of the reference to a mobile station via a secondbase station; receiving, by the first base station, a random accesspreamble including the signature indicated by the reference from themobile station in instances where the reference indicates a signaturewithin the first signature group; and receiving, by the first basestation, a random access preamble including the signature within thesecond signature group from the mobile station in instances where thereference indicates a signature within the first signature group.
 7. Themethod according to claim 6, further comprising: after receiving arandom access preamble including the signature within the secondsignature group from the mobile station, sending, by the first basestation, a contention resolution message to the mobile station.
 8. Themethod according to claim 6, wherein, the first base station does notsend the contention resolution message to the mobile station ininstances where the number indicates a signature within the firstsignature group.
 9. A mobile station for random access to a first basestation at a time of handover, the first base station being configuredwith a first signature group and a second signature group, the mobilestation comprising: a receiver, configured to receive a messageincluding a reference used for random access from a second base station;a scheduling section, configured to determine whether the referenceindicates a signature within the first signature group or the secondsignature group; and a transmitter, configured to send a random accesspreamble including the signature indicated by the reference to the firstbase station in instances where the number indicates a signature withinthe first signature group, and to perform a contention resolutionprocessing in instances where the reference indicates a signature withinthe second signature group.
 10. The mobile station according to claim 9,wherein the reference is determined by the first base station andtransmitted to the second base station.
 11. The mobile station accordingto claim 9, wherein contention resolution processing comprises:receiving a contention resolution message from the first base station.12. The mobile station according to claim 11, wherein contentionresolution processing further comprises: sending a random accesspreamble including a signature randomly selected within the secondsignature group to the first base station.
 13. The mobile stationaccording to claim 11, wherein the receiver is further configured to donot receive the contention resolution message from the first basestation in instances where the reference indicates a signature withinthe first signature group.
 14. A base station for random access at atime of handover, the base station being configured with a firstsignature group and a second signature group, the base stationcomprising: a signature managing section, configured to determine anumber used for random access, wherein the number indicates a signaturewithin the first signature group or the second signature group; atransmitter, configured to send a message including the number to amobile station via another base station; and a receiver, configured toreceive a random access preamble including the signature indicated bythe number from the mobile station in instances where the numberindicates a signature within the first signature group, and to receive arandom access preamble including the signature within the secondsignature group from the mobile station in instances where the numberindicates a signature within the first signature group.
 15. The basestation according to claim 14, wherein, the transmitter is furtherconfigured to send a contention resolution message to the mobile stationafter receiving a random access preamble including the signature withinthe second signature group from the mobile station.
 16. The base stationaccording to claim 14, wherein, the transmitter is further configured tonot send the contention resolution message to the mobile station ininstances where the number indicates a signature within the firstsignature group.